I haven't have much time to do tracking/continuous shooting tests, however one thing stands clear: My Sigma 70-200, f2.8 HSM Macro lens does not perform well with K-3 II. The lens works as expected with K-30.

With FPS priority for AF Action Shooting on K-3 II, it's as if the lens want's to go out of focus and it doesn't matter much what the subject is or if it's stationary or moving.

My 70mm Limited however is rock solid in comparison - I can do FPS priority AF Action Shooting and it will nail more shots in focus than my K-30. The out of focus shots are are just slightly off focus, though still too far off if I take my "wedding photographer" glasses on.

I have a strong gut feeling about the K-3 II AF system being unreliable. I already mentioned I had one evening with the Sigma 18-35 with horrible results followed by another evening with much better results... and then bad results again. Last night I did some quick tests using Pentax 70mm Limited, 21mm Limited and Sigma 17-50mm @ 17 and 50mm. They all failed under low light situation here at home.

Today, the 70mm nailed all shots, the 21mm nailed nailed most while the Sigma @ 17mm failed at pretty much all shots but nailed pretty much all @ 50mm.

I've had all these three lenses for at least 3 years and done tons of shots with them and can say for sure the lenses have not previously behaved this way - that is except, when I shoot using my other K-30 body. Over time it's AF became worse and I could get severely out of focus shots at 117mm and small apertures - I had it turned in twice, last year and the service guys claimed there were no problem - I turned it in with the lens as well, still no problem, they said - no that I trust them, I actually do not.

Another thing: I've before taken photos of my monitor, though it's been a while since I did it in a similar fashion the last time: With the Sigma 18-35mm mounted on K-3 II and focus set to minimum distance, the camera/lens wouldn't even bother to focus or it would focus as if the subject is at minimum focusing distance and then give up. the target was an an white image with a bunch of long and thin black bars displayed fullscreen on my 30inch monitor, shot from about 2 meters away. No problem what so ever with Sigma 17-50mm and my other lenses. The 18-35 had no problem if the lens was already focused a little bit in.

Another thing: I've before taken photos of my monitor, though it's been a while since I did it in a similar fashion the last time: With the Sigma 18-35mm mounted on K-3 II and focus set to minimum distance, the camera/lens wouldn't even bother to focus or it would focus as if the subject is at minimum focusing distance and then give up. the target was an an white image with a bunch of long and thin black bars displayed fullscreen on my 30inch monitor, shot from about 2 meters away. No problem what so ever with Sigma 17-50mm and my other lenses. The 18-35 had no problem if the lens was already focused a little bit in.

Is it possible that your Sigma 18-35 started so out of focus, coupled with being an f/1.8 lens, that the PDAF sensor had nothing to pick up to drive the focus?

This sounds like a defect to me. You might want to exchange the camera.

It's possible the f/2.8 high-precision points (or their associated optics) are bad. Does this reproduce with any points other than the middle three in the center column of the AF array? If not, than this is the issue and your camera is defective.

I'm not experiencing any AF trouble of this sort with my K-3 II and DA 50mm f/1.8 lens.

I tried with more of my lenses and the Pentax 35mm Macro behaved in similar way. It focuses from either end but if the distant meter is set from about 0.2m to about 0.17m, then it would consistently focus right next to 0.17m.